Unfortunately, this material responds to variations in humidity. 

 When the humidity falls, the material contracts and pulls away 

 from the glass. For a number of years, the variation in humidity 

 has caused the contraction and expansion of this material on 

 some of the models to such an extent that it has separated com- 

 pletely from the glass, a condition readily visible in many plant 

 cross-sections. A constant temperature and humidity system has 

 been installed to reduce this damage to a minimum. By stabiliz- 

 ing the collection, it will prevent further depreciation. 



There is another aspect of the creation of these models which 

 should be mentioned. In the late 1890's, as Rudolph continued 

 modelling plants, he became dissatisfied with the quality of the 

 glass available to him, and he decided to make his own glass. 

 Consequently, he had several glass furnaces installed in his stu- 

 dio and proceeded to produce glass himself from the raw mate- 

 rials. He further proceeded to work toward another objective: to 

 improve on one of the techniques involved in his work. As men- 

 tioned above, many of the plant and all of the invertebrate 

 models were made of clear glass and colored by applying a min- 

 eral material with glue and/ or gum. This surface material could 

 be scratched off and was susceptible to changes in humidity. It 

 was Rudolph's intention to make the models in such a way as to 

 eliminate these shortcomings. He prepared the parts of a plant of 

 clear glass; instead of applying the glue-gum-pigment mixture, 

 he added powdered glass of the desired color to the surface and 

 re-heated the parts in the flame, thus fusing the clear and the 

 powdered glass. It was a somewhat chancy procedure, inas- 

 much as the second heating of the part sometimes caused the 

 glass to fracture. When successful, this process assured a more 

 durable model. Rudolph was successful in this endeavor to a 

 certain extent but we cannot say how many of the models were 

 completed in this manner. We do know that some of the cross- 

 sections were definitely made in this way, for it can be visually 

 determined. Other models, however, may have been made in this 

 way, but it cannot be detected merely by visual examination. 

 After the application of powdered glass and fusion, the piece so 

 treated appeared glassy; to overcome this deficiency, he coated 

 the surface with a material to eliminate the glassy shine. 



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